Calderasib

Calderasib (MK-1084) is Merck's oral, selective KRAS G12C inhibitor, a pill that chemically locks down a broken cell-growth switch found in a slice of lung and colorectal tumors [1]. Merck is running it through at least four Phase 3 programs: KANDLELIT-012 in first-line KRAS G12C metastatic colorectal cancer combined with cetuximab and mFOLFOX6 [2], KANDLELIT-004 with pembrolizumab in PD-L1-high NSCLC [3], KANDLELIT-015 with durvalumab in NSCLC [4], and MK-1084-013 with pembrolizumab in adjuvant resected NSCLC [5]. The drug enters a class where Amgen's sotorasib and Bristol Myers Squibb's adagrasib are approved but commercially underwhelming, and where Revolution Medicines just reset expectations for the broader RAS space with positive overall survival data on a pan-RAS inhibitor [6]. Merck is not late to KRAS so much as it is betting that combination-first development, especially with its own Keytruda franchise, can extract value the monotherapy-first peers could not.

Calderasib is a novel investigational compound, not approved in any indication. Public FDA designation records do not show breakthrough, fast track, orphan, or RMAT status for calderasib as of mid-2026. Merck has at least five active calderasib trials recruiting: Phase 1 dose escalation in KRAS-mutant solid tumors (NCT05067283, n=830) [7], Phase 2 expansion in advanced solid tumors (NCT07209111, n=150) [8], and the four Phase 3 programs listed above [2][3][4][5]. The most-watched near-term catalyst is KANDLELIT-004, which tests calderasib plus pembrolizumab against pembrolizumab monotherapy in 1L PD-L1 TPS ≥50% NSCLC, a setting where any combination has to beat one of the most effective drugs in oncology. No formal timeline guidance for primary readouts has been issued by Merck on calderasib specifically. Given recruitment status, pivotal readouts are more plausibly 2027 to 2028 than 2026. Investors should also expect a Phase 1/2 efficacy update at a major medical meeting (ASCO or ESMO) before the Phase 3 reads, which is the next real data event.

KRAS is one of the most-mutated genes in human cancer. It works as a molecular on-off switch: when KRAS is on, it tells the cell to grow and divide; when off, the signal stops [9]. Certain mutations jam the switch in the on position, so the tumor cell keeps multiplying without permission. The G12C mutation specifically swaps a glycine for a cysteine at position 12, and that cysteine contains a sulfur atom that drugs can chemically bond to, creating a covalent handle that nature did not put there. Calderasib uses that handle. It binds KRAS only when the protein is in its inactive (GDP-bound) state and traps it there before it can flip back on. Sotorasib (Lumakras) and adagrasib (Krazati) hit the same cysteine the same way and are FDA-approved in KRAS G12C NSCLC, so the mechanism is fully validated at the regulatory level [10]. The hard question is durability. Tumors find resistance routes (acquired KRAS mutations, bypass through RTK signaling, MET amplification) within months, and that is why every serious developer in this class, Merck included, is building combination regimens rather than chasing monotherapy approvals.

KANDLELIT-012 (NCT06997497) is the colorectal pivotal: calderasib plus cetuximab plus mFOLFOX6 versus standard of care chemotherapy in 1L KRAS G12C unresectable or metastatic CRC [11]. The combination choice is deliberate. Adagrasib needed cetuximab to look competitive in KRYSTAL-10, so Merck is conceding the monotherapy lane in CRC and going straight to chemo plus EGFR blockade plus G12C. KANDLELIT-004 (NCT06345729, n=600) is the highest-leverage trial commercially: calderasib plus pembrolizumab versus pembrolizumab monotherapy in PD-L1 TPS ≥50% 1L NSCLC, PFS primary [3]. KANDLELIT-015 (NCT07554339, n=310) pairs calderasib with AstraZeneca's durvalumab in NSCLC, PFS primary [4]. MK-1084-013 (NCT07431827, n=400) is the adjuvant resected stage IIA-IIIB N2 KRAS G12C NSCLC trial, DFS primary [5]. The breadth is the strategic statement. Merck is spending heavily, in parallel, across CRC, 1L PD-L1-high NSCLC, IO-combo NSCLC, and adjuvant NSCLC. No detailed Phase 2 monotherapy efficacy data on calderasib has been disclosed in peer-reviewed form, which is the principal information gap right now.

Our model estimates a 38% chance this drug is eventually approved. It starts from the historical base rate for Phase 3 drugs in this area (about 57%), then adjusts using ten facts about the trial and sponsor. What moves the number most: it is helped by more secondary endpoints than usual and the sponsor's strong record of getting drugs approved; it is held back by weak or limited earlier-phase results and a randomized design. The other facts land near average for this stage, so they leave the estimate roughly where the base rate put it.

Efficacy risk is the dominant concern. KRAS G12C inhibitors have not produced the durable single-agent responses seen with EGFR or ALK inhibitors. Median PFS for sotorasib in 2L NSCLC is roughly 5 to 6 months, and adagrasib looks similar [12]. Calderasib will only matter clinically if combinations meaningfully extend that, and combination benefit in this class is unproven at Phase 3 OS level. Safety risk is real but manageable. Class-level signals include hepatic enzyme elevation, GI toxicity, and QT effects; sotorasib carried FDA scrutiny around hepatotoxicity in combination contexts. Adding calderasib to mFOLFOX6 plus cetuximab stacks overlapping toxicities (neuropathy, neutropenia, rash, transaminitis), and IO combos add immune-mediated adverse events. Execution risk is moderate. KRAS G12C is only about 3 to 4% of mCRC, so KANDLELIT-012 enrollment will be slow and biomarker-driven. Commercial risk is the underrated one. Sotorasib peaked at roughly $300M annual run-rate, and adagrasib has not been a commercial breakout either despite a clean approval [13]. Daraxonrasib's pan-RAS overall survival win in PDAC (RASolute 302, ASCO 2026) does not directly cannibalize G12C revenue in NSCLC or CRC where G12C-selective remains standard, but it does reframe how payers and oncologists think about the value of mutation-selective covalents in any indication where pan-RAS becomes viable [6].

Worth watching, with one trial doing most of the work. KANDLELIT-004 is the signal. If calderasib plus pembrolizumab beats pembrolizumab monotherapy on PFS in PD-L1 TPS ≥50% 1L NSCLC, that is a market-redefining result because pembrolizumab mono is current standard of care and is hard to beat. A positive PFS hazard ratio under ~0.65 with clean safety would force a label expansion conversation and reset the IO+TKI combo playbook for the whole field. KANDLELIT-012 in CRC is lower conviction because the chemo plus cetuximab backbone is already active and the absolute G12C CRC population is small. KANDLELIT-015 (with durvalumab) and the adjuvant trial are supportive but secondary. Merck's $65B revenue base and 89% Phase 3 conversion rate mean execution risk is low; the question is whether the molecule itself has differentiated PK or CNS penetration versus divarasib (Roche) and glecirasib (BeiGene, NDA filed 2025). Next check-in: any conference (ASCO or ESMO) where Merck might release Phase 1/2 calderasib monotherapy or combo response data, plausibly late 2026 or 2027. Until then, the priors are set by sotorasib, adagrasib, and the daraxonrasib RAS(ON) story.